Electronic engine speed control system



Dec. 15, 1953 F. F. oFFNER ELECTRONIC ENGINE SPEED CONTROL SYSTEM 3 Sheets-Sheet l Filed Aug. 27, 1947 INVENTOR. JM Z Om, EVP @5W e, PMM,

Dec. 15, 1953 F. F. oFFNER 2,662,372

ELECTRONIC ENGINE SPEED CONTROL SYSTEM Filed Aug. 27, 1947 3 Sheets-Sheet 2 NVENTR.

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Dec. 15, 1953 F. F. oFFNER 2,662,372

ELECTRONIC ENGINE SPEED CONTROL. SYSTEM Filed Aug. 27, 1947 3 Sheets-Sheet 5 JNVENTOR. 35ML J 0%.

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Patented Dec. 15, 1953 UNITED STATES PATENT OFFICE ELECTRONIC ENGINE SPEED CONTROL SYSTEM Franklin F. Olner, Chicago, Application August 27, 1947, Serial No. 770.372

Claims. (Cl. Gil-39:28)

This invention relates to electrical control systems and in particular to systems of the null-seeking type for controlling the magnitude of a variable in accordance with a reference standard.

The invention is considered to be of general utility and the particular nature of the variable to be controlled will accordingly depend upon the type of apparatus to which the invention is ap plied. Thus depending upon the apparatus, the variable may be temperature, pressure, liquid level, speed, etc. The invention is well suited for speed control of rotating members such as engines and other types of power plants and is herein illustrated and described in its application to engines of the jet propulsion type. However, it is to be understood that the invention is not limited to this particular application but is deemed to also cover such modifications and uses as come within the scope of the invention as expressed in the appended claims.

An object of the invention is to provide an improved electrical control system of the nullseeking type in which a direct current voltage variable in proportion to the magnitude of the variable to be controlled is continually compared with another direct current voltage but which is of constant magnitude that serves as a standard or reference voltage; any deviation in the variable from the magnitude desired to be maintained brings about a corresponding change in the vvariable voltage and the resulting difference between the latter and the reference voltage serves as a voltage signal that is nrst amplified and then applied to the control device, by which the magnitude of the variable is controlled, in such manner as to remove the deviation and reestablish a balance between the variable and reference veltages.

Another object of the invention is to provide for automatically controllingr the rate at which an engine is accelerated from idling to various running speeds and in such manner that the acceleration rate is automatically increased as the speed of the engine increases. In the case of jet engines, this is particularly important to prevent overheating of the turbine, combustion chamber, and exhaust nozzle components especially at low speeds where the amount of air available for coolingr the parts is limited.

Another object is to provide an adjustable engine speed control of the null-seeking type which introduces an additional control quantity variable with another variable of the engine such as for vexample engine temperature or altitude by which the speed of the engine is automatically changed in accordance with predetermined changes in the variable irrespective of the nature of the relation between the engine speed and reference quantities.

Another object ls to provide for automatically decreasing the speed of an engine in the event that its temperature exceeds a preselected maximum value considered safe for normal operation.

Yet another object of the invention is to regulate fuel flow to the engine as a function of enigne speed so as to place a limit on the rate of fuel flow to be permitted for each different speed of the engine.

Another object oi the invention is to provide an improved electronic amplifier for extremely low direct current voltages.

The foregoing and other objects and advantages to be derived from the invention will become more vapparent from the following description and accompanying drawings which illustrate the invention as applied to jet engines.

In the drawings, Fig. 1 is a block diagram showing the principal control components of a preferred embodiment of the control system; Fig. 2 is a schematic circuit diagram of the system shown in Fig. 1; Fig. 3 is another block diagram illustrating a modified form of the invention; Fig. 4 is a schematic circuit diagram of the modified embodiment shown in Fig. 3; and Fig. 5 is a speed-time curve illustrating a typical speed change characteristic of the engine as established by the control vsystem in accordance with this invention.

Referring now to Figs. 1 and 2, it will be seen that the control system includes a small generator I having an armature la coupled by shafting to the jet engine '2 so as to be driven by the latter at a speed proportional to that of the engine. The output voltage from armature la which will vary with its speed is split by a voltage divider comprising series connected resistors 4, 5 which are grounded at the junction 6 therebetween into relatively positive Yand negative potentials. The negative potential appearing across resistor 4 is passed through a filter l o1" the LC type comprising inductance 'la and condensers 1b, 1c to the negative voltage terminal 8. The positive potential appearing across resistor 5 is passed through a lter 9 comprising resistance 9a and condenser 9b to the positive voltage terminal I0. Filters 'l and 9 serve to remove commutator ripples and unwanted voltage frequencies caused by engine torsional vibrations and `hence a steady direct current potential is delivered to the speed voltage terminals 8 and I 0. The potential at terminal 8 will be negative with respect to the ground connection; the potential at terminal IB will be positive with respect to ground; and the two potentials will vary in proportion to the speed of the engine 2.

For setting the engine 2 to run at not less than a preselected minimum speed, and for also adjusting the control system to operate at Various stabilized cruising speeds, a bridge circuit II is provided. The latter includes resistor arms I2, I3 having the junction point C therebetween connected via lead I4 to the negative speed voltage terminal B. Also included in the bridge are two parallel connected slide wire potentiometers I6, I1. One terminal of each of the latter is grounded at I8 and the other terminal of each is connected via lead 2| through resistor 22 and the field Ib of the engine driven generator I to the positive terminal of a source of direct current voltage 23, the negative terminal of this voltage source being grounded as shown by the conventional symbol. The voltage source 23 supplies a reference voltage to the potentiometers I6 and I1, and the latter serve respectively as a speed selector control adjustable by the operator to Vary the cruising speed of the jet engine, and as a minimum speed control which is preset to fix the minimum engine speed to be permitted.

By examination of the circuit in which the eld coil Ib of the tachometer generator I is connected in series with the parallel arranged speed control potentiometers I 6, I1, it will be evident that any uctuation in the voltage source 23 is reflected equally in the generator field and the speed control potentiometers. Thus, for example, should the supply voltage decrease, it will lower the reference voltage at the potentiometers i6. I1 but it will also correspondingly lower the negative and positive speed signal voltages appearing at terminals 8 and I0. As operation of the control system depends upon a balance between the signal and reference voltages taken from the potentiometers I6, I1, for any speed, the advantage of the interconnections between the generator eld and potentiometers is obvious for since both the signal and reference voltages will vary an equal amount there will be no variation in balance, and therefore engine speed, regardless of any reasonable change in the D. C. supply voltage.

The bridge H is completed by another resistor arm 24 having the same resistance value as arm I3 that is connected between an end terminal D of resistor I3 and one side of a rectifier 25 to f which the slidable contact arm on potentiometer I6 is also connected through a resistor 26. The other side of rectifier 25 connects with the slidable contact arm on the minimum speed control potentiometer I1. The convention observed with respect to the rectier 25 as illustrated is that electron flow is in the direction of the arrow.

The bridge II functions to compare the relative magnitudes of the negative speed voltage from generator I and the reference voltage (which is positive) as set on the two potentiometers i6 and I1, and to deliver a potential at point D on the bridge whose sense (i. e. positive or negative) and magnitude will be proportional to the sense and magnitude of the difference between the speed and reference voltages. A damping condenser 21 connected between points C and D of the bridge Il differentiates the negative speed voltage. This derivative voltage is applied at terminal D and serves to prevent the 4 control system from overshooting at the engine speed set on potentiometer I6 when the engine is accelerated from an idling speed. It also serves to reduce any hunting eiect at the selected engine speed.

The minimum speed control potentiometer I1 and the cruising speed control potentiometer I6 function conjointly with the rectifier to furnish the positive reference voltage to the bridge II. If the voltage setting of potentiometer I6 is positive with respect to the setting of potentiometer I1, rectier 25 will not conduct and consequently, the positive reference voltage to be compared with the negative speed voltage by bridge II will depend upon the setting of potentiometer I6. However, should the take-olf voltage at potentiometer I6 be less positive than that of potentiometer I1, then current will now through rectifier 25 and the reference voltage applied to the bridge will depend solely upon the setting of the minimum speed control potentiometer I1.

The direct current signal output at point D denoting a diierence between the instant speed at which the engine 2 is running and a desired speed as represented by the magnitude of the reference voltage, is interrupted by means of a vibrator type of switch S which includes stationary contacts 28, 23 and a grounded vibrating reed contact therebetween driven by a Vibrator coil 3|. The signal voltage from point D is periodically grounded through stationary contact 29 and reed contact 36 producing a square wave signal, the alternating component of which is passed through condenser 34 and applied to one plate of a unidirectional conducting device 35 that can be a disc rectifier of the selenium type, through an adjustable tap on resistor 3B. Rectier 35, depending upon its bias, will either block the signal or allow it to pass to the input of the signal amplifier 31. The convention observed with respect to the rectifier 35 and also with respect to rectifier 82 to be subsequently explained is that electron flow is in the direction of the arrow. The manner in which the rectifier bias, referred to as the crossover voltage is derived, is described later in another section.

Assuming that the bias on rectifier 35 is such as will pass the voltage signal coming from point D on bridge II into the amplifier 31, the signal is amplified in a high gain pentode single ended amplifier stage 3B, a phase inverter stage 39, and two push pull stages 43, 4 I. The ampliiied square wave signal is then passed to the primary 44a of output transformer 44. The output in the transformer secondary 44h is rectified back into a direct current signal through vibrator switch SI comprising stationary contacts 45, 46, connected to the opposite ends of the secondary winding 44h and vibrating blade contact 41 driven from coil 48. The contacts of the switch SI may be structurally combined with those of switch S so that both blade contacts 33 and di are driven by one coil and therefore operate synchronously. However, synchronous operation of the two switches may also be obtained in the manner shown in the drawings by making the switches structurally independent and synchronizing them electrically. Coils 3| and 48 of the two structurally independent Vibrators S and SI are connected to the positive terminal 49 of a direct current voltage source and operate synchronously through the agency of a choke coil 50 and resistor 5I combination. The amplified direct current signal is applied via connections 52, 5B to the series connected coils 54a, Bib of a proportional solenoid 54.

A fraction of the amplified output signal taken across resistor 55 and integrated by resistor 56 and condenser .'il is fed back through resistor 56a and connection 5B to the output side of the speed control bridge Il and combines with the latter to further improve the stability of the system. IThe fwd-back signal is always opposite in polarity to that of the speed signal which developed it and hence adds to the stabilizing effect of condenser 21 in reducing any tendency on the part of the system to hunt above and below any selected value engine speed.

Armature 54e of the proportional solenoid, biased to a neutral position by springs 54d, 54e operates the piston member Gla oi' a pilot valve 6I. Cylinder Glb of the valve contains a central high pressure Huid inlet 62, low pressure outlets 63 at each end lead to a sump, and high pressure fluid outlets 65, t4 leading to the opposite ends of the cylinder 65D of servomotor t5. The piston 55a of the servomotor actuates a vane 66 in the fuel injection line 61 leading to engine 2 to control the rate of fuel flow to the combustion chamber.

The proportional solenoid 54 also includes a toroidal permanent magnet 54j, for setting up a magnetic field which when combined with the magnetic field produced electromagnetically by the signal current in coils 54a, 54h, produces axial movement of armature 54o in one direction or the other from its neutral position iixed by the loading springs 54d, 54e dependent upon the sense of the current flow through these coils which in turn depends upon the polarity of the rectified output signal at vibrator Si.

The corresponding motion imparted to valve piston Gla places the high pressure line 62 in communication with one end of cylinder 65h of servomotor 65 and the other end of cylinder 55h in communication with one of the low pressure outlet lines 83 causing servomotor piston 65a to shift axially and change the setting of the vane 66 in the fuel injection line 61. Movement of armature 56e in the opposite direction from its neutral position in response to an output control signal of opposite polarity will of course have an opposite edect on the setting of the fuel control vane 66.

The proportional solenoid 54 is of a type for which a more detailed explanation of its construction and principles of operation may be found in my no-pending application, Ser. No. 68,862, led Jan. 3, i949, and the application of Stanley G. Best led October 28, i947, new Patent No. 2,579,723.

As the speed of the'engine 2 is increased or decreased as the case may be by a change in the setting of the fuel control vane Et, the negative speed voltage at terminal 8 will undergo a like change until it rematches the new setting of the speed control potentiometer I6 which gave rise to the signal that led to the change in engine speed, at which time the signal from bridge ll will be reduced to zero. Thus the system operates on the well known null principle.

In the jet type of engine, it has been found necessary to place a limitation on the maximum rate at which the engine is to be permitted to be accelerated, especially when bringing the engine up from idling to a selected cruising speed in order to prevent the temperature of the blading of the turbine element as well as that oi the combustion chamber of the engine from reachingr ills dangerously high levels. An acceleration control circuit 10 serves this function and produces an acceleration rate signal which automatically increases exponentially with the speed of engine 2. At low engine speeds, the amount of air taken in by the engine is quite small. Hence the cooling effect derived from this air for the turbine and combustion chamber is correspondingly of a low order. However, as the engine speed is increased, the increased air flow at the higher speeds allows a greater amount of engine torque output to be generated without overheating, which consequently permits of a greater acceleration rate. The acceleration circuit 'Ill operates below the crossover speed as determined by a "crossover circuit to be later explained. Above the crossover speed, the acceleration control is rendered inoperative and the speed control il takes over.

Included in the acceleration circuit are resistors 'li and 'l2 connected in series between the negative speed voltage terminal 8 and the adjustable contact arm on potentiometer I1. Resistor 'l2 is provided with an adjustable tap 'i3 to which is connected one end of resistor 1A, the other end of the latter being grounded. Resistor M together with an associated adjustable tap 15 constitute a potentiometer for taking oit a suitable fraction of the voltage existing between the tap point H on resistor 12 and ground. The voltage take off from tap 15 which as hereinafter explained will be negative is applied through resistors 76, 11 to a circuit junction point F where it is combined algebraically with a positive acceleration signal voltage developed by difierentiating the positive speed voltage signal appearing at terminal I0 at condenser 18 to produce the acceleration signal. From junction F, the acceleration signal is passed through resistor 18 to contact member 28 on vibrator switch S.

As with the speed control circuit previously eX- plained, the D. C. acceleration signal is converted by the vibrating grounded arm 30 into a square Wave signal, the alternating component of which is applied through condenser 8i! and an adjustable tap on resistor 8i to one side of a rectifier 82. Provided the bias on rectifier 92 is of the proper polarity, which is determined by the voltage of the cross-over circuit, the acceleration signal will be passed through it into the input of amplifier 31.

Assuming now that the engine is idling and it is desired to bring it up to a selected stabilized speed, speed control i6 is advanced to a selected speed and the tap on resistor 'l2 adjusted to provide a small negative voltage at point H and hence also at point F. At this time the crossover voltage will be such as to allow the small negative signal to pass through amplifier 31 and thence to the fuel control valve to cause the latter to open slightly and therefore increase the engine speed slightly.

It should be noted at this point that because of the relative phase reversal in the signal l input and output circuits of the amplifier at the vibrators S and SI which function to alternately chop the speed and acceleration signals at the amplifier input, the polarities of the two signals must be the reverse of each other to eiect a like change in the setting of the fuel control valve. In the illustrated construction, a positive signal at the output of the bridge H results in an increase in fuel flow to the engine and hence the acceleration signal to obtain the same result must be negative. The system can of course be reversed so that negative signals from bridge I I and positive signals from the acceleration circuit 1U result in an increase in fuel now.

As the engine speed increases, the negative voltage at point H increases and hence the voltage at point F will become more and more negative as it continues to combine algebraically with the derivative of the positive speed signal from condenser T8 resulting in higher and higher negative acceleration signal voltages which in turn result in an increasingly greater acceleration rate in the engine. The change in voltage at point F is thus an exponential one and the initial rate of acceleration is determined by the setting of resistors 12 and 14, a greater initial negative voltage at point H resulting in higher initial acceleration.

The acceleration curve produced by the acceleration circuit 10, assuming the fuel vane E6 operates quickly enough to keep the input always substantially in balance, is

where w is the engine speeed, B is a constant determined by the setting of the initial acceleration control resistor 12, and A is a constant determined by the setting of the acceleration rate increase control potentiometer 13.

It should be noted that should the negative speed voltage at terminal 8 drop enough below the minimum speed setting that point H became positive, the input signal to amplier 8 would be such as to call for a deceleration whereas an acceleration is desired. To prevent this from occurring, a small negative voltage is inserted at point G between resistors 16 and 11 through a rectier 83, resistors 8l and 81a and battery 88.

The bias voltage for rectifers 35 and 82 which determines whether or not the signals from the speed control circuit II or the acceleration control circuit shall pass to the input of the amplifier 31 is obtained from a control unit 90 which for convenience can be called the crossover circuit. This circuit, which acts as a switching device to fix the point in the speed curve of engine 2 at which the amplifier input signal changes over from the acceleration control 10 to the speed control Il, is comprised of resistors Si, 92 and 93 connected in series between points B and C of the speed control bridge I I. Resistor 92 has an adjustable tap and a lead 94 extends from the latter to a point intermediate resistors 36 and BI.

It will be recalled from the previous description that the alternating components of the square Wave speed and acceleration signals are passed by the condensers 34 and 80, respectively. The bias voltage from the crossover circuit applied over lead 94 to a point intermediate the resistors 3E and 8l is superimposed upon the signals passed by condensers 34, 80. If the crossover voltage appearing at the input side of rectifier 35 is negative, current can flow only through this rectiiier into the input circuit of amplifier 31, and such current will fluctuate in accordance with the alternating component of the speed signal. Conversely when the crossover voltage becomes positive, current can now only through rectifier 82 into the ampliiier input and such current will fluctuate in accordance with the alternating component of the acceleration signal. t should now be clear that the rectifiers 35 and 82 function as switching means which pass or block the speed and acceleration signals, respectively in dependence upon the polarity of the crossover voltage.

At speed circuit balance, the potential at the tap of resistor 92 will be slightly negative. This negative potential applied to rectiers 35 and 82 will permit any signals from the speed control circuit II to pass through rectifier 35 but is sufficient to cut oif at rectifier 82 any signals coming from the acceleration circuit 10. If the speed control potentiometer IS is advanced to raise the positive reference potential, the crossover voltage at the point between resistors 36, 8| will become positive which then cuts ofi rectifier 35 and allows only the acceleration signals from circuit 13 to pass through rectifier 82 to the input of amplier 31; this condition prevails until the engine speed is suiciently high, at which point the crossover voltage cuts 01T rectifier 82 and allows only the signals from the speed circuit I I to pass into amplifier 31 through rectifier 35, When the engine is to be reduced in speed by retarding potentiometer IE, the crossover voltage will become more negative and the signals from bridge II control the engine speed.

The control circuit as shown in the drawings has been applied to jet engines of the type known as Model J-33--15l and a number of test runs made. A speed-time curve for a typical run is shown in Fig. 5. It will be noted from this curve that in the initial stages of the increase in engine speed, the change in the engine acceleration, denoted by the tangent to the curve at any point, was held down. After a run of two seconds, however, the curve begins to rise increasingly more steeply which is indicative of increasingly greater engine acceleration until at the end of Sil seconds the engine reached the cross-over" point at a speed of 9000 revolutions per minute. At this point, the negative signals from the acceleration circuit 1l became blocked by rectifier 82, and the positive signals from the bridge Il took over to control the remaining increase in engine speed until final speed balance, as determined by the setting of potentiometer I3, was attained. It will be noted that at an engine speed of 10,500 R. P. M., the curve begins to taper oi at an increasing rate as a result of the damping action provided by condenser 2l, the engine reaching the selected and stabilized final speed of approximately 11,i50 R. P. M. after a run of approximately sixteen seconds.

In operating engines of the jet type, it has been found that sonic control should be maintained over the maximum temperatures, considered permissible in the tail pipe 2a from which the hot propulsion gases are expelled. The present electronic type of engine control well suited for this purpose and tail pipe temperature limitation is obtained by superposing a signal voltage variable with the temperature on the control signals supplied by the speed and acceleration circuits I I, 1D at the input to amplifier 3l. The polarity of the signal is such that when a preselected critical temperature in the tail pipe is exceeded, the fuel vane 5S will be moved towards a more closed position thus reducing the fuel input to the engine which in turn is reflected by the desired decrease in temperature of the propelling combustion gases at the tail pipe.

Limitation of the tail pipe temperature is derived from a temperature bridge Sl, that includes as one arm thereof a temperature sensitive resistor unit 93 located in the tail pipe 2a of the engine. The other three arms of this bridge are constituted by resistor 99, series connected resistors |00, and |02 with an adjustable tap on resistor IIlI, and series connected adjustable resistors |03, |04. Point N of the temperature bridge 91 is grounded and positive potential is supplied to point L of the bridge over conductor from the direct current voltage source 23. Connected in series between the bridge diagonals M and O are, another vibrator switch unit S2 mechanically coupled to the reed contact 38 on Switch S so as to operate synchronously there with comprising a. central vibratory reed contact |06 and two outer stationary contacts |58, |08, the primary |I2a. of transformer I I2 and a single pole double throw switch H3. In one position of the blade contact of switch I|3 legended balance, the transformer primary I I2a is connected through to point 0 of the bridge, and in the other position legended "Run, primary winding |I2a is connected to the tap on resistor arm IGI. The transformer secondary IIEb is connected in a series circuit that includes rectiiier H3, resistors I I4, I I5, the blade contact |06 on vibrating switch S2 and stationary switch contact |08,

After the bridge 91 has been calibrated by means of the adjustable resistor arms |03, |04 with the switch I I3 in the Balance position, the latter is thrown to the Run" position and the tap on resistor arm IOI adjusted to a point at which the bridge 91 will become balanced at a resistance value of the temperature sensitive resistor 98 that corresponds to the maximum temperature to be permitted in the tail pipe 2c. This may be l500 F. for example. It will now be evident that during the time that the engine 2 is being run, a temperature in the tail pipe 2a lower than the permissible maximum selected by the setting of resistor |0I will cause the bridge to become unbalanced in one direction to develop a voltage of one polarity at the bridge diagonal terminals MO', While a tail pipe temperature higher than the selected maximum will cause the bridge to be unbalanced in the opposite direction and reverse the polarity of the voltage appearing at terminals MO.

The direct current output voltage from bridge 01 converted into a square wave signal by the vibrator switch S2 and passed through the transformer primary II2a, induces a corresponding voltage in the secondary I|2b which when contacts |06, |08 of switch S2 are closed is applied across the rectifier I I3 and resistors IM, I I5.

The bridge 91 is so arranged that Whenever a temperature above the selected permissible maximum prevails in the tail pipe 2a, the voltage developed in the transformer secondary I I2b will have such a polarity as to enable it to be passed by rectifier IIS. On the other hand, should an unbalance condition in the bridge 8l be attributable to a drop in tail pipe temperature below the value at which the bridge is caused to be balanced, the voltage induced in the transformer secondary ||2b will be of opposite polarity and hence be blocked by rectifier I I3.

Thus for tail pipe temperature lower than the selected maximum, no signal can get through the rectiiier IIB, while for temperatures higher than such maximum, rectifier II3 conducts and passes the voltage signal into resistors H4, H5. This signal, taken oil resistor I I4 by an adjustable tap and lcd via conductor IIS to the input of ampliiier 31, produces an output signal of such polarity as will result in a movement of the fuel Vane 66 to a more closed position irrespective of the fact that at such instant the signals delivered by the acceleration circuit 'I0 or the speed circuit I I may iii) l0 be calling for a. more open position of the fuel vane B6.

A modied embodiment shown in Figs. 3 and 4 uses a relay type of control for the fuel valve and operates the latter by means of a reversible motor whose rotative direction and angular displacement is determined by the sense and magnitude of the output signals from the amplifier 3l. This modified form of the control system also provides for limitation of fuel flow as a function of the speed of engine 2.

The system shown` in Figs. 3 and 4 is identical with that shown in Figs. 1 and 2 up to the terminal points .tc-41:, and hence need not be redescribed. Starting with terminals x-, the ampliiied square wave signal voltages from the speed control circuit I I. the acceleration control 10. the temperature control 91, and a further signal from a fuel limitation control |46 which will be described later in more detail, are further amplified in a balanced push-pull amplifier stage IIB and passed to the primary |I9a of output transformer IIS. A suitable fraction of the output from the transformer secondary IISb is taken from resistor |23 and rectified by the vibrator switch S3. This switch which operates synchronously with switch S, includes a vibratory blade contact I2| mechanically coupled to the blade contact 3|) on switch S and stationary contacts |23, |24. Dependent upon the circuit connections, stationary contacts |23, |24 will be correspondingly positive and negative, or reversed, with a given polarity of input signal to the ampliiier 31. If a positive speed signal from the speed control II, or a negative acceleration signal from the acceleration control 'I0 is received at the input to the amplier 31, a negative signal will develop causing one of the two relays |25, |26 to open for a brief period, which in turn will cause the fuel vane |21' to open and the engine speed to increase. If the signals are reversed, the other relay opens causing the fuel vane |27 to move towards a more closed position thus decreasing the engine speed.

The coils of relays |25, |26 are connected in the anode circuits of tubes |28, |29. respectively. These relays operate normally closed, i. e. a zero or positive signal at the tube signal grids I 28a, I29c will allow sufiicient anode current to flow to keep the relay coils energized. However, with the appearance of a negative signal at stationary contact |23 of vibrator switch S3, condenser I 30 will be charged at a rate depending upon the resistance of the pulse frequency control resistor |20. The control grid |28a will be biased negatively by the charge on condenser |30 until the anode current through tube |28 becomes sunlciently low to permit the contacts of relay to open. At the moment of opening, condenser with its negative charge, is connected via relay contacts I25a and lead I3I to the pulse length control resistor |32 in the grid circuit of amplifier stage ||8 and allowed to discharge. Since a negative voltage already exists across resistor |32 due to the grid current of tube IIB, the rate of discharge of condenser |35 will depend upon the magnitude of this voltage as well as the time constant of the RC network of resistor |32 and condenser |35. The magnitude of the voltage drop existing across resistor |32 depends in turn upon the magnitude of the signal voltage applied to the input of amplifier 3T. With a large input signal, the voltage across resistor |32 will be larger and a longer time will be taken before the potential across condenser I3!! is suicently low to allow the relay |25 to reclose. Stated in a different manner, a longer puise length will exist with a large signal than with a smal! one, resulting in a greater change in engine speed per pulse with large signal bccausc the relay will be opened longer.

Relay |25 operates in the same manner as described above for relay |25 when a. negative signal appears on stationary contact |24 of the vibrator switch Si. With. a rio-signal condition. the anode current of tube |23 is suiiicient to keep this relay energized. However, the appearance of a negative signal causes condenser |33 to charge at the rate determined by the setting of resistor Ie thus biasing the control grid |2Sa negatively and reducing the anode circuit current until relay |25 drops out. Condenser |33 then discharges through resistor |32 via relay contacts |2Ea and connection |3|.

Assuming that the jet engine 2 is already running at the desired speed set on speed control potentiometer l5, no signal will appear on the vibrating blade contact |2| of vibrator switch S3 and hence the alternate engagement of this contact with the stationary contacts |23, |24 will have no change effect upon the bias on the signal grids of the tubes |28, |29.

Under such a condition, the energizing coils of both relays |25 and |26 will receive sufficient current to maintain their respective movable contact members in the positions shown in the drawing, and both brushes of the armature |34a of fuel valve motor |34 are thus connected to the negative side of a direct current source of power' |35 via relay contacts l25d and |2Ed. No current will thus flow through the armature. The eld |341) of motor |34 is separately excited by the power source |35 and with a no-current condition prevailing in armature |34a, the latter will stand still and hence not disturb the setting of the fuel vane |21 to which it is coupled by worm I3 and sector gear |31.

Should now the setting of the speed control I6 be advanced so as to cause a negative signal to appear at stationary contact |23 each time it engages the blade contact |2|, the resulting charging of condenser |33 in the negative direction and its subsequent discharge will cause relay |25 to pulse open and closed at a rate determined by the iii-circuit values of the charge and discharge control resistors and |32 thus intermittently changing one of the motor armature connections through relay contacts |d from the negative to the positive side of the power source which causes current to ilow through the armature and rotate it in such a direction as will move the fuel vane |21 to a more open position. The engine 2 will now speed up until the negative speed voltage from generator matches the new setting of the speed control potentiometer I3 at which time the voltage signal at the amplier output transformer IIS will be reduced to zero which stops the pulsing action of relay |25 allowing it to resume a closed position and stopping current flow through the armature of fuel control motor If on the other hand, the speed control potenticmeter I5 is retarded which results in the appearance of negative signal at switch contact |24 each time it is engaged by vibratory contact |2|. condenser |33 is charged negatively and results in periodic deenergization and reenergization, i. e. pulsing of relay |26 thus changing the connection for the motor armature |34a through relay contacts |26d from the negative to the positive side of power source |35. Current will now be caused to ilow through armature |3401 in the opposite direction to rotate in the reverse direction and move the fuel vane |21 to a more closed position. Engine 2 will now decrease its speed until the reduced speed voltage matches the reduced reference voltage at potentiometer I6. At such time, the signal at output transformer ||9 is once again reduced to zero allowing relay |26 to remain reenergized and cut off the ilow of current in the motor armature |34a.

As a safety precaution to prevent the fuel control motor |34 from running uncontrolled should one of the relays |25, |26 fail to reclose when the signal voltage which initiated its opening disappears, a safety relay |38 is used. Contacts |38b and |380 of this relay are inserted in the connections |39, |40 between the fuel control motor armature |34a and relay contacts |25d, |26d. So long as the coil of relay |33 is energized from source |4| through the series connected relay contacts |250, |26c, which is the condition existing when relays |25, |26 are also V energized, the circuit to the motor armature |34a will remain closed through relay contacts |3811, |38c. However, in the event that either one of the relays |25, |26 should fail to properly reclose after the signal is removed, causing the energizing circuit to relay |33 to remain open at either relay contacts |25c or |260 for an undue length of time, the contacts of relay |38 will open after a. predetermined length of time dependent upon the capacity of condenser |42 which discharges into the coil of relay |33 through resistor |43, and cut orf the flow of current to motor arma ture |34a. The capacity of condenser |42 is of course made suiciently large so that relay |33 will not open for the normal biiefpe1iod interruptions of its energizing circuit caused by normal opening operations of relays |25, |28 in re-l sponse to engine speed signals transmitted through the signal amplifier 31. A pilot lamp |44 to indicate to the operator that the speed control system for engine 2 is not operating, may be provided, and is shown connected so as to be lighted from a source |45 through relay contacts |38a when the latter close upon deenergization of relay |38.

In some instances, it is desirable to limit the flow of fuel to the engine 2 as a function of engine speed. That is, to provide for a predetermined maximum fiow of fuel to be permitted for each different engine speed. The control in the Fig. 4 circuit for accomplishing this, and which is designated generally by reference numeral |45 is seen to include a potentiometer I4? having its adjustable arm |41a in driving relation with the armature |34a of the fuel valve motor 34, so that the voltage output from potentiometer |41 will be a function of the angular position of armature |34a. The negative speed voltage, obtained at point C of the speed control bridge and fed via conductor |48 is combined with the positive voltage taken off potentiometer |41 through re-sistors |49, |50 and applied to rectifier IBI.

When operation of the control system is nor` mal, the ratio of the setting of the potentiometer |41 to the negative speed voltage will be such that the two voltages always result in a negative voltage on the plate of rectiiier |5|. Under this condition, rectiner |5| blocks the transmission of current and hence the fuel control |46 remains inoperative. This means that ihmm I3 the ratio of engine speed to fuel new is high enough that fuel flow limitation is not introduced.

Should, however, the potentiometer |41 be advanced too high for the negative speed voltage at any particular speed, then the sum of the voltages will switch to a positive value and result in a signal being passed through rectifier |5| into the input to amplifier 31 through coupling resistor I5?. and condenser |53. As with the other signals introduced to the amplier, the fuel flow control signal is periodically grounded and thus interrupted through contacts |54, of a vibrator switch S4 driven by coil |56 operating synchronously with the other three vibrator switches S, S2 and S3. The signal through the rectifier |5I duly appears at the output transformer |9 and is in such a phase as to function through the ensuing operation of relays |25, |26 to rotate the armature Hed of the fuel valve motor Iii?` in the direction as will reduce the amount of i'uel ilowing past vane |21 and also reduce the setting of the potentiometer |41. Thus for each speed of engine 2 there is a maximum fuel iiow which is permitted. At values of fuel flows higher than the maximum, a signal is generated which reduces the flow to the permissible maximum.

In conclusion, it will be appreciated that the specific control arrangement which has been described and shown in this application may be departed from without, however, departing from the spirit and scope ci the invention as dened in the appended claims. As an example, While an acceleration responsive signal is utilized to control the initial acceleration of the engine or other rotating member' up to the cross-over speed, and a velocity responsive signal used thereafter to control the remainder of the change in speed desired. the basic cross-over type of control described in this application will function equally as Well to effect the switching of voltage signals derived as functions of parameters of engine operation other than the velocity and acceleration characteristics specifically described herein, such as for example engine temperature.

I claim:

l. In an electrical control system of the automatically rebalancing type for controlling the speed of a rotating member wherein a direct current voltage proportional to the speed of the member and produced by a generator coupled to the member is continually compared with a reference voltage of selected magnitude, and any difference therebetween sensed to provide a voltage sig` al that is utilized to alter the speed of the member to restore balance between the two voltages, the improvement which consists in employing the cutout of a potentiometer as the referenco voltage and connecting the potentiometer and iield ci the generator to the same source of voltage whereby any change in the voltage of the source is reflected equally in the output voltages of both the generator and the potentiometer.

2. In a system of speed control for an engine, in combination, means producing a speed voltage variable with the engine speed, means providing a reference voltage, means comparing said voltages to derive a signal voltage whose polarity and magnitude is determined by the sense and magnitude of the difference between said voltages, reversible means responsive to said signal voltage and actuated in one direction or the other dependent upon its polarity, a fuel control actuated by said reversible means for changing the rate of fuel admission to the engine, means actuated by said reversible means for producing a. second reference voltage variable with the displacement of said reversible means, means comparing said second reference voltage with said speed voltage to derive a second signal voltage, means at the output of last said comparing means for passing said second signal voltage only when said second reference voltage exceeds said speed voltage, said passed second signal voltage having a polarity such as will effect a reduction in the rate oi fuel flow to the engine, and means applying said passed second signal voltages to said reversible means.

3. In an electrical speed control of the automatically rebalancing type, means producing a voltage signal whose magnitude and polarity are determined by the magnitude and sense of the departure in speed ci.' a rotating member from a given speed, an electrically controlled speed adjusting device for the member responsive to said signal for altering the speed of the member in such sense as to reduce the signal to zero, means controlled by an operating parameter of the rotating member other than speed for producing a second signal upon an increase in the magnitude of said other operating parameter above a preselected magnitude, and circuit means applying said second signal to said speed adjusting means, said second signal having a polarity such as tc effect a reduction in speed of the rotating member.

4. ln an electrical speed control of the automatically rebalancing type for a rotating member, means producing a voltage signal Whose magnitude and polarity are determined by the magnitude and sense of the departure in speed of the rotating member from a given speed, an electrically controlled speed adjusting device for the member responsive to said signal for altering the speed of the member in such sense as to reduce the signa1 to zero, means controlled by temperature of the member for producing a second signal upon a rise in temperature of the member above a preselected maximum level, and circuit means applying said second signal to said speed adjusting device, said second signal having a, polarity such as to effect a reduction in speed of the member.

5. In an electrical speed control of the automatically rebalancing type for a rotating member, means producing a voltage signal whose magnitude and polarity are determined by the magnitude and sense of the departure in speed of the member from a given speed, an electrically controlled speed adjusting device for the member responsive to said signal for altering the speed of the member in such sense as to reduce the signal to zero, means providing a second signal variable in accordance with the temperature of the member, said signal having a zero value at a preselected maximum temperature level and circuit means applying said second signal to said speed adjusting device when it is of polarity resulting from temperature above preselected maximum temperature level. and blocking said second signal when of reverse polarity, said second signal when applied having a polarity such as to effect a reduction in speed of the member.

6. In an electrical speed control of the automatically rebalancing type for a rotating member, means producing a voltage proportional to the speed of the member, means providing a reference voltage of a selected magnitudel means electrically combining said voltage to produce the difference therebetween, an electro-controlled speed adjusting device for the member responsive to the difference voltage for altering the speed of the member in such sense as to reduce the difference voltage to zero, a comparison circuit generating a voltage which varies in accordance with changes in magnitude of an operating parameter oi the member other than speed, said comparison circuit being balanced at a predetermined magnitude level of the parameter, means in the output of said circuit for passing only such output voltages as are attributable to unbalance caused by a change in the magnitude of the parameter in a predetermined sense, and circuit means applying said passed output voltages to said speed adjusting device.

7. In a system of speed control for a rotary machine, in combination, means producing a speed voltage variable with the machine speed, means providing a reference voltage, means comparing said voltages tc derive a signal voltage Whose polarity and magnitude is determined by the sense and magnitude of the diiiercnce between said voltages, reversible means responsive to said signal voltage and actuated in one direction or the other depen-:lent upon its polarity, means actuated by said reversible means for changing' the speed of the machine, a temperature bridge including a temperature sensitive element as one branch thereof responsive to temperature or the machine, said bridge being balanced at a predetermined machine temperature limit, means in the output of said bridge for passing only those output voltages as are attributable to bridge unbalance caused by machine temperatures in eX- cess of said temperature limit, said passed output voltage having a polarity such as will eflect a reduction in speed of the machine, and means applying said output voltage to said reversible means.

3. In a system of speed control for a `iet type engine in which the engine power is delivered rearwardly through an exhaust tail pipe, in combination, means producing a speed voltage variable with the engine speed, means providing a reference voltage, means comparing said voltages to derive a signal voltage whose polarity and magnitude is determined by the sense and magnitude of the difference be wcen said voltages, reversible means responsive to said signal voltage and actuated in one direction or the other depencling upon its polarity, means actuated by said reversible means for changing the speed of the engine, a temperature bridge including a temperature sensitive element as one branch thereof responsive to the temperature in the tail pipe, said bridge being balanced at a predetermined tempcraure limit in the tail pipe, means in the output of said bridge for passing only such output voltages as are attributable to bridge unbalance caused by tail pipe temperatures in excess of said temperature limit, said passed voltage having a polarity such as will effect a reduction in speed of the engine, and means applying said output voltage to said reversible means.

9. In a speed control for a rotating member, an electrically controlled speed adjusting device for the member, means providing' a first electrical signal which is a function of one operating parameter of the member, means providing a second electrical signal which is a function of a second operating parameter of the member, connections between said signals and said speed adjusting device, and switching means in said connections for said signals controlled in accordance with the speed of the member, said switching means being actuated at a predetermined speed of the member to cut oil said first signal from said speed adjusting device and simultaneously cut in said second signal.

10. In a speed control for a rotating member, means producing a irst electrical signal which is a function of the speed of the member, means producing a second electrical signal which is a function of the acceleration of the member, an electro-controlled speed adjusting device for the member responsive to said signals, and switching means for said signals controlled in accordance with the speed of said member, said switching means being actuated at a predetermined speed of the member to cut oi said rst signal from said speed adjusting device and simultaneously cut in said second signal.

11. In a speed control for a rotating member, means providing a first electrical signal which is a function of one operating parameter of the member, means providing a second electrical signal which is a function of second operating parameter of the member, an electrically controlled speed adjusting device for the member responsive to said signals, means producing a cross-over signal varying with speed of the member, and means controlled by said cross-over signal for blocking transmission of one of said signals to said speed adjusting device below a selected magnitude of said cross-over voltage and for similarly blocking transmission of the other of said signals above such magnitude of the crossover voltage.

12. In a combined acceleration and speed control of the automatically rebalancing type for a rotating member, means producing a voltage proportional to the speed of the member, means producing a voltage proportional to the acceleration of the member, means providing a first reference voltage, means electrically combining said first reference and acceleration voltages to derive a first voltage signal, a second reference voltage, means electrically combining said second reference voltage and speed voltage to derive a second voltage signal proportional to their difference, an electro-controlled speed adjusting device for the member responsive to said signals, and switching means for said signals controlled in accordance with the speed of said member, said switching means being actuated at a predetermined speed of the member to cut off said first signal from said speed adjusting device and simultaneously cut in said second signal.

13. A control system as defined in claim 12 wherein said rst reference voltage includes a voltage component increasing with speed of the rotating member so that the resultant acceleration increases with speed.

14. In a combined acceleration and speed control of automatically rebalancing type for a rotating member, means producing a voltage proportional to the speed of the member, means producing a voltage proportional to the acceleration of the member, means providing a first reference voltage, means electrically combining said rst reference and acceleration voltages to derive a first voltage signal, a second reference voltage, means electrically combining said second reference voltage and speed voltage to derive a second voltage signal proportional to their difference, an electro-controlled speed adjusting device for the member responsive to said signals, circuit means combining said reference and speed voltages to produce a crossover voltage increasing with speed of the member, and signal block- 17 ing means controlled by said crossover voltage for blocking transmission of said second signal to said speed adjusting device below a selected magnitude of said crossover voltage and for simllarly blocking transmission of said ilrst signal above said voltage.

15. A combined acceleration and speed control as definedl in claim 14 wherein said signal blocking means is constituted by a unidirectional conducting device in each signal circuit to which said crossover voltage is applied to render the said devices conductive and non-conductive, respectively, at the said selected magnitude oi' the crossover voltage.

16. In a combined acceleration and speed control for a rotating member, means producing a direct voltage proportional to the speed of the member, means dividing said voltage into speed voltages of opposite polarity, means producing first and second direct reference voltages, means diflentiating one of said speed voltages, circuit means electrically combining said diilerentiated speed `voltage with said i'lrst reference voltage and the other one oi said speed voltages to produce a iirst voltage signal, means electrically combining said other speed voltage with said second reference voltage to produce a second voltage signal of opposite polarity to that of said rst signal, a signal amplier, a vibrator switch at the input to said amplifier for alternately connecting said first and second signals to the amplier, a second vibrator switch at the amplifier output operated synchronously with said first switch for rectifying the amplified signals, said amplified signals being of like polarity, an electro-controlled speed adjusting device for said rotating member responsive to the rectified ampliiler output signals, and switching means responsive to speed of the rotating member for cutting oil said rlrst signal at a predetermined speed oi' the member and simultaneously cutting in said second signal.

17. In a combined acceleration and speed control of the automatically rebalancing type for a rotating member, means producing a voltage proportional to speed of the member, means dividing said voltage into positive and negative speed voltages, means producing ilrst and second reference voltages, means diierentiating one oi said speed voltages, circuit means electrically combining said differentiated speed voltage with said rst reference voltage and the other of said speed voltages to produce a first voltage signal, circuit means combining the said other speed voltage with said second reference voltage to produce a second voltage signal having a, polarity opposite to that of said rst signal, a signal amplifler, a vibrator switch at the input to said amplifier for alternately connecting said first and second signals to the amplifier, a second vibrator switch at the amplifier output operated synchronouslv with said first switch for rectlfying the amplified signals, said amplied signals being of like polarity, circuit means electrically combining one of said speed voltages with said reference voltage to produce a cross-over voltage increasing with speed of the member, an electro-controlled speed adiusting device for the rotating member responsive to the amplified output signais, and signal blocking means controlled by said cross-over voltage for blocking transmission of said second signal to said speed adjusting device below a selected magnitude of the cross-over voltage and for similarly blocking transmission of said first signal above such voltage.

1B. In a combined acceleration and speed control oi' the automatically rebalancing type for a rotating member, means producing a voltage proportional to the speed of the member, means dividing said voltage into positive and negative speed voltages, means producing iirst and second reference voltages, means diiierentiating said positive speed voltage, circuit means electrically combining said differentiated speed voltage with said iirst reference voltage and said negative speed voltage to produce a iirst voltage signal, circuit means combining said negative speed voltage with said second reference voltage to produce a second voltage signal having a, polarity opposite to that of said iirst signal, a signal amplifier, a vibrator switch at the input to the amplifier for alternately connecting said first and second signals to the amplifier, a second vibrator switch at the amplifier output operated synchronouslly with said first switch for rectifying the ampliied signals, said amplified signals being of like polarity, circuit means electrically combining one of said speed voltages with said reference voltage to produce a cross-over voltage increasing with speed of the member, signal blocking means controlled by said cross-over voltage at the amplier input for blocking transmission of said second signal below a selected magnitude of the cross-over voltage and for similarly blocking transmission of said first signal above such voltage, and an electro-controlled speed adjusting device for the rotating member responsive to the rectiiied output signals.

i9. In an electrical control system, the combina'tion comprising a pair of input circuits each adapted to have impressed thereon a signal voltage. and switching means connected in each of said circuits to control passage of the signal voltage therethrough, said switching means being arranged upon application thereto o a variable control voltage to pass one of said signal voltages and block the other in accordance with the instantaneous value of said control voltage.

20. In an electrical control system, the combination comprising a pair of input circuits each adapted to have impressed thereon a signal voltage having a pulsating characteristic. and rectilers individual to and connected in each of said input circuits, said rectiers being arranged for conduction alternatively upon application thereto of a common variable control voltage to pass one or the other of said signal voltages.

2l. In an electrical control system, the combination comprising a pair of input circuits each adapted to have impressed thereon a signal voltage having a pulsating characteristic, and rectiiiers connected in each of said input circuits. said rectifiers being oppositely poled and alternatively conductive upon application thereto of a common control voltage of reversible polarity to thereby pass one or the other of said signal voltages.

22. In an electrical control system, the combination comprising a pair of input circuits each adapted to have impressed thereon a direct signal voltage, an amplifier for said voltages. said ampliiier including synchronously actuated interrupter means at the input and output sides thereof for respectively converting the signal voltages applied to said input circuits from direct to pulsating at the ampliiier input and reconverting the same from pulsating to direct at the amplier output, and rectier means connected in circuit with each of said signal voltages between the input and output interruptor means of said 19 amplier, said rectilers being arranged for conduction alternatively upon application thereto of a common variable control voltage to pass one of said pulsating signal voltages and simultaneously block the other.

23. In an electrical control system, the combination comprising a pair of input circuits each adapted to have impressed thereon a direct signal voltage, an amplier for said voltages, said amplifier including synchronously actuated interrupter means at the input and output sides thereof for respectively converting the signal voltages applied to said input circuits from direct to pulsating at the amplifier input and reconverting the same from pulsating to direct at the amplifier output, and rectifier means connected in circuit with each of said signal voltages between the input and output interrupter means, of said amplier, said rectiers being oppositely poled and alternatively conductive upon application thereto of a common control voltage of reversible polarity to thereby pass one of said pulsating signal voltages and simultaneously block the other.

24. In an electrical control system, the combination comprising a pair of input circuits each adapted to have impressed thereon a direct signal voltage, an ampliiier for said voltages, said amplifier including synchronously actuated interrupter means at the input and output sides thereof for respectively converting said signal voltages from direct to pulsating at the amplier input and reconverting said voltages from pulsating to direct at the amplifier output, and a signal selector for blocking transmission of one or the other of said signal voltages through said amplifier, said signal selector including rectiers individual to and connected in each of said input circuits on the amplifier side of said input interrupter means and which are oppositely poled for conduction alternatively upon application thereto of a common control voltage of reversible pola- 20 rlty to thereby pass one or the other of said pulsating input voltages.

25. In a speed control of the null-seeking type for an engine, means providing rst and second signals which vary respectively in accordance with different parameters of engine operation. a cross-over control for selecting one or the other of said signals, a device actuated by the selected signal for adjusting the fuel iiow to the engine in such manner as will tend to nullify the selected signal, and means responsive upon attainment of a preassigned condition of engine operation for actuating said cross-over control to cut off one of said signals from said fuel flow adjusting device and cut in the other signal to said device.

FRANKLIN F. OFFNER.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 746,047 Dodge Dec. 8 1903 1,187,795 Treten JunefJ A191e 1,580,368 Boddie Apr, 1926 1,686,792 Black Oct. 9, 1928 1,743,545 Helpbringer Jan. 14, 1930 1,751,225 Ashbaugh Mar. 18, 1930 2,247,166 Edwards et al June 24, 1941 2,336,232 Doran Dec. 7, 1943 2,428,702 Elliot Oct. '7, 1947 2,442,300 Liston May 25, 1948 2,492,472 Fortescue Dec. 2'?, 1949 OTHER REFERENCES Theory of Servo Systems with Particular Reference to Stabilization, by A. L. Whiteley, pages 601-603, Journal of Electrical Engineers, December 1946.

Diierentiating and Integrating Circuits, by James G. Clarke, pages 13B-142, Electronics, November 1944. 

